I planted some nannyberries some 10 years ago. There's a borer that attacks the tops of the plants when they are young, boring down thru the main leader and killing off the top growth. Once the plant gets over about 4', the problem disappears. Only 2 of the 6 plants have made it past that height (they are now 10-15' tall). 2 have died, 2 are still struggling. No defoliation and no leaf issues. Any pest ideas or treatment ideas? I've looked and not found this pest in the literature. Thanks!
I am wondering if you have had an infestation of viburnum leaf beetle. They lay their eggs into the new growth late in the season and these eggs overwinter inside the twigs hatching the next season, leaving dead twigs at the tops of the plants. It can be so severe that it can seriously damage and kill plants. The Nannyberry, Viburnum lentago can be a host for this insect and the damage would appear like a borer. This insect also will feed on the leaves severe enough to weaken the plant. it is the larval stage that is most susceptible to insecticides. It came into Washtenaw around 2010 and has devastated many viburnums. There are two articles that I recommend you look at one shows the damage on foliage and twigs and the other talks about pesticides. Check out these articles and especially look at the picture of the twigs where the eggs have been laid. If this does not fit what you are seeing you may want to get a few samples of twigs into the Washtenaw MSU Extension office for diagnosis. Call 997-1678 for information on having a sample evaluated. See the following sites: